Most nurses seem to agree that the Littmann is one of the better stethoscopes. They come in different models and qualities. I have a Classic II Littmann and it retails for around $100. There are some Littmans that are cheaper, and others of higher quality that probably cost four or five hundred.
Don't despair if you're on a tight budget though. The Sprague is a fairly good quality stethoscope that cost about $20 14 years ago when I was in nursing school, its probably a bit more now.